In a wireless system, the receiving node typically needs to receive a minimum power level per tone of the transmitted information burst. A Hybrid Automatic Repeat Request (HARQ) protocol allows retransmission of the information burst by the transmitting node and combining the received information bursts in the receiving node to effectively increase the received power per tone. The transmitting node can gain about 3 decibels (dB) with the same power per tone by retransmitting the original information burst. If the information burst is retransmitted 3 times, i.e., a total of 4 transmissions of the same information burst, the transmitting node can gain about 6 dB compared to a single transmission of the information burst.
FIG. 1A illustrates a prior art timeline 100 of the communication exchanges between a base station and a mobile station operating in accordance with HARQ. The prior art timeline 100 shows five frames 102, 104, 106, 108, and 110. In frame x 102, the base station sends a HARQ uplink Medium Access Protocol Information Element (HARQ UL MAP IE) 120 to the mobile station. The HARQ UL MAP IE 120 indicates to the mobile station to send particular information to the base station. In frame x+1 104, the mobile station sends a HARQ UL transmission (HARQ UL TX) burst 122 of the particular information to the base station in response to the HARQ UL MAP IE 120.
The base station receives the HARQ UL TX burst 122 and processes it to check for any errors. The received HARQ UL TX burst 122 is assumed to have an error in this example and the base station sends another HARQ UL MAP IE 124 in frame x+3 108 to indicate to the mobile station to retransmit the particular information again. After receiving the UL MAP 124, the mobile station sends a HARQ UL retransmission (HARQ UL reTX) burst 126 of the particular information to the base station in frame x+4 110.
The time interval between HARQ UL TX burst 122 and HARQ UL reTX burst 126 is denoted as HARQ Round Trip Time (RTT) 132 and is assumed to be 3 frames for the purpose of illustration. The HARQ RTT is the minimum time between a transmission of a particular information burst and the retransmission of the particular information (or a new transmission of another information burst) in the same HARQ channel.
Repetition coding is another technique that can increase reliability of the transmission. For example, a repetition coding scheme with a repetition factor of two, repeats two times for each bit that is to be transmitted and the number of slots required for transmission is doubled as shown in FIG. 1B. If the transmitting node is limited by the total power that it can transmit, repetition coding does not help in increasing the reliability of the transmission. This is because the power per tone needs to be halved in order to transmit twice as many tones, assuming that the transmission requires the maximum total power when no repetition coding is used.